1. Field of the Invention
This invention relates generally to electrochemical fuel cells. More specifically, the present invention relates to an electrochemical fuel cell which has at least one fluid distribution layer which decreases in permeability from an inlet to an outlet of the fuel cell.
2. Description of the Related Art
Electrochemical fuel cells convert reactants namely, fuel and oxidant fluid streams, to generate electric power and reaction products. Solid polymer fuel cells typically employ a membrane electrode assembly (“MEA”) consisting of a solid polymer electrolyte or ion exchange membrane (“PEM”) disposed between two electrode layers comprising electrocatalysts, namely a cathode and an anode. The membrane, in addition to being an ion conductive (typically proton conductive) material, also acts as a barrier for isolating the reactant streams from each other. The MEA also typically includes fluid diffusion layers adjacent to the electrode layers for purposes of distributing reactants evenly to the electrodes.
At the anode, the fuel stream moves through the porous anode diffusion layer and is oxidized at the anode electrocatalyst layer. At the cathode, the oxidant stream moves through the porous cathode diffusion layer and is reduced at the cathode electrocatalyst layer to form a reaction product. The location of the electrocatalyst generally defines the electrochemically active layer.
In electrochemical fuel cells, the MEA is typically interposed between two substantially fluid impermeable separator plates (anode and cathode plates). The plates typically act as current collectors and provide support to the MEA. The plates may have reactant channels formed therein and act as flow field plates providing access of the fuel and oxidant to the porous anode and cathode diffusion layers, respectively, and providing for the removal of product water formed during operation of the cells.
Water management issues are critical in PEM fuel cell operation and humidification of the membrane is required to maintain optimal performance. As the water content of the membrane falls, it loses the ability to transport protons, its electrical resistance increases, fuel cell performance decreases and membrane failure may occur. To ensure adequate humidification of the membrane, one or both of the reactant streams supplied to the fuel cell stack are typically humidified. Such humidification is commonly provided by an external humidification system, however, such external systems increase the cost, complexity and size of the fuel cell system.
U.S. Pat. No. 6,451,470 and Canadian Patent Application No. 2,342,825 disclose the use of gas diffusion electrodes (each comprising a “gas diffusion barrier” or “GDB”), having a gas permeability gradient perpendicular to the membrane, rather than, or in addition to, uniformly porous anode and cathode substrates (each a “gas diffusion layer” or “GDL”), to inhibit the diffusion of water away from the membrane. The use of such gas diffusion electrodes enables fuel cell operation without external humidification of the reactants.
Additionally, the conditions in an operating PEM fuel cell vary significantly across the electrochemically active area of each electrode. For example, in a conventional fuel cell, as the oxidant is consumed, water is produced, the total gas pressure normally decreases and the oxidant partial pressure decreases. This results in a greater current density in the first third to half of the cell as compared to the latter half of the cell. Performance of the cell is likely limited by the high current density region, thereby resulting in an overall voltage lower than what would be obtained if the current density were uniformly distributed across the cell. High current density may also result in increased local temperatures that tend to lead to greater material degradation. Higher temperatures may also result in a decrease in the humidity at the inlet which can increase the likelihood of transfer leaks developing across the membrane and cause a loss of performance. This latter effect can be exacerbated if there is little or no humidification of the inlet reactant streams. While the inlet portion of the cell is likely to be too dry, the outlet portion of the cell is likely to have too much water which can result in localized flooding, uneven performance and increased mass transport losses. Thus, the requirements and desired properties of the fuel cell electrode will vary across the fuel cell.
U.S. Pat. No. 5,840,438, which is incorporated herein by reference, discloses the performance benefits of increasing the fluid permeability of a fuel cell electrode substrate between a reactant inlet and outlet, thereby facilitating the removal of product water. U.S. Patent Application No. US 2003/0039876 and Japanese Publication No. 2001-135326 also disclose an electrode substrate having a permeability gradient which increases from the fuel cell inlet to outlet.
Similarly, International Publication No. WO 00/31813 discloses the use of an additional perforated plate interposed between a separator plate and an adjacent porous fluid distribution layer, wherein the perforations in the additional plate increase in size between the inlet and outlet.
Although there have been advances in the field, there remains a need for fuel cell systems which are able to manage the accumulation of product water and maintain the level of humidification necessary for optimal performance. The present invention addresses these needs and provides further related advantages.